The Acoustic beam profile or shape in an Acousto-Optic (AO) device is defined by the electrode pattern on a piezoelectric transducer bonded to the device body. The radiation pattern from common rectangular shaped piezoelectric transducers produces SINC function, sin(2.theta.)/(.theta.)!.sup.2, pattern in the interaction plane, where .theta. is the angle from transducer normal in the AO interaction plane. This SINC function pattern has extensive side-lobe structure. The diffraction of light from an AO device can be considered to take place from the angular spectral distribution of the transducer radiation in the AO interaction plane. See Adrian Korpel, "ACOUSTO-OPTICS", Marcel Dekker, Inc., New York, pp. 77-93, 1988. The acoustic side-lobes generated due to the transducer geometry, manifest through AO interaction as ghost images when an image is formed or as some structure, known as side-lobe, in spatial, temporal or frequency domain depending upon the AO device application. The SINC function radiation pattern from a rectangular transducer can be mathematically predicted from the spatial Fourier transform theory, See John N. Lee, Editor, "Design Issues in Optical Processing", Cambridge University Press, pp. 78-136, 1995. The effect of any discontinuity in the transducer pattern on the image and side-lobe performance of an AO device can be similarly predicted using spatial Fourier transform of the discontinuity. If a discontinuity causes a side-lobe in the acoustic radiation pattern in the AO interaction plane then a ghost image can occur in an AO device for imaging application or side-lobe may occur in spatial, temporal or frequency domain depending upon the AO device application.
The relation between optical diffraction pattern and transducer geometries in AO devices has been extensively analyzed and reported. See Dennis R. Pape, Peter A. Wasilousky, and Mike Krainak, "A high performance apodized phased array Bragg cell", SPIE Vol. 789 Optical Technology for Microwave Applications III, p116, 1987. When a rectangular transducer as shown in FIG. 1a is replaced with one of the commonly used apodized transducers as shown in FIGS 1b, 1c, and 1d then side-lobes in the acoustic radiation pattern are reduced or suppressed. The apodized transducers have edges oriented away from the normal to the AO interaction plane. This orientation produces Fourier components due to the edge discontinuity out of the interaction plane having large Bragg mismatch and hence absence of optical diffraction from the edge and discontinuity effects.
We have built AOTFs(Acousto-Optic Tunable Filters) with diamond shaped apodized transducers as shown in FIG. 1b and obtained images with negligible ghost images. On the other hand an AOTF with rectangularity shaped transducer produces severe ghost images. Ghost images severely degrade the spatial and spectral resolution of an AOTF based imager. A ghost free image is desirable in an AOTF based imager. Therefore, apodized transducer is commonly required for most AOTFs.
An improvement in the AOTF spatial resolution is possible with a larger optical aperture. However, a larger aperture AOTF requires larger transducer size. The radiation resistance of a piezoelectric transducer is inversely proportional to the area of piezoelectric transducer. A large transducer of an AOTF causes the radiation resistance to become quite small. The resistive part of practical reactive components used for impedance matching a large area AOTF transducer can become much larger than the radiation resistance in such a case. This results in significant reduction of the transducer bandwidth and electrical to acoustic conversion efficiency. A common technique to improve the bandwidth of a large area transducer is to section the transducer and connect the sections in series(See Joel F. Rsenbaum, "Bulk Acoustic Wave Theory and Devices", Artech House, Boston, Mass., p. 235, 1988) as shown in FIG. 2 for a rectangular transducer. A sectioning and series connection technique for improving the bandwidth of large area transducers which are encountered in the AOTF applications is shown in FIG. 3 as reported in Dennis R. Pape, Peter A. Wasilousky, and Mike Krainak, "A high performance apodized phased array Bragg cell", SPIE Vol. 789 Optical Technology for Microwave Applications III, p116, 1987, for the apodized transducers used in the prior art. Series connections of the transducer segments raises the transducer impedance value so that the impedance matching elements with high Q can be selected to allow a wider transducer bandwidth. Alternatively, sectioning of an apodized transducer for series connections can be accomplished in a similar manner with an equal area for each section as shown in FIG. 4a-4c.
Various patents relate to the subject invention but fail to obviate the problems previously discussed. See U.S. Pat. Nos. 4,348,609 and 4,760,358 to Inoue; 4,918,349 to Shiba et al.; 5,136,266 to Niitsuma; 5,155,406 to Cho et al.; and 5,369,382 to Arvanitis.